I'm not too sure about that. This story has a lot of red flags for me. My partner grew up with 3 half brothers and his mother often insulting her exes for cheating. He grew up with a decent amount of resentment. The main parent figure ended up being his stepfather. OP didn't say that she sat and had a good convo with her kid and just said "screw that cheater", which is selfish. It's not about the parents, put things aside and let the children choose for themselves. The kid might know how upset the mother is at the other family, and not want to tell op that they like their step sibling. We need more info.. I'm also curious as to why he seems to have the full security, what happened there (OP said she had visitation)?